Mobile radio communication systems rely upon radio frequency signals to transmit data. The quality of a received signal in such a system depends upon the strength of a carrier signal relative to any noise signal that is introduced during transmission or by the circuitry of the communication system. The relative strength of the received signal is affected by the strength of the transmitted signal and by the distance between the receiver and the transmitter. As the distance increases, received signal strength tends to deteriorate.
In addition, the signal does not usually travel solely along a direct path from the transmitter to the receiver. Although the direct path is one potential propagation path, other possible paths exist. For example, the signal may reflect off of objects that are large with respect to the signal wavelength, such as the side of a building, thereby causing the signal to travel along a reflected path. The signal may be refracted by a knife-edge surface, such as the corner of a building, thereby causing the signal to travel along a refracted path. Finally, an object that is small relative to the wavelength, such as a traffic light, may cause the signal to scatter, thereby causing the signal to travel to the receiver along a scattered path.
Thus, the signal may travel from the transmitter to the receiver along a direct path, a reflected path, a refracted path, a scattered path, or some combination thereof. When the signal travels along multiple paths, each of the multiple coherent signals travels a different distance between the transmitter and receiver. As a result, each signal has a somewhat random phase and amplitude. The phase and amplitude of the overall received signal results from the vector addition of the multiple coherent signals.
In some instances, this combination results in an improvement in the strength of the received signal (constructive interference). In other cases, the received signal strength is degraded (destructive interference) by the multiple path (multipath) propagation.
In the mobile environment, multipath fading occurs as the receiver moves from a zone of constructive interference to a zone of destructive interference. As the vector sum of the multiple coherent signals varies over time, there are significant variations in the strength of the overall carrier signal with respect to the strength of the noise signal. Although these variations may exist when the transmitter-to-receiver distance is static, multipath fading tends to increase with increases in the relative velocity of the transmitter and receiver as the receiver travels between zones of constructive and destructive interference. Because of the significant variations in received signal strength, multipath fading will sometimes cause the strength of the noise signal to instantaneously exceed that of the carrier signal. When this occurs, the received signal may experience a 360 degree phase rotation, which will cause clicks or pops in the received audio signal.
One way to diminish the effects of multipath fading is to employ a diversity reception system. A diversity reception system uses a plurality of receivers and selects between receivers to generate an improved overall signal. The receivers in a diversity reception system are individually coupled to antennas that are spatially separated from one another. When one receiver is experiencing a fade caused by the multipath propagation of the carrier signal, another receiver may have better reception because of the spatial separation of the receivers. By selecting the receiver with the best reception, the overall audio signal produced by the diversity reception system can be improved.
In one type of diversity reception system, the system selects the active receiver according to the RF input signal level of each receiver. Use of receiver RF input signal level as the selection criterion, however, is not always effective because the receiver RF input signal level does not provide an accurate indication of signal quality at low RF input signal levels. A diversity reception system may also rely on the signal-to-noise ratio of the receiver output signals to select the active receiver. Use of the signal-to-noise ratio of the output signal as the selection criterion, however, is also problematic because the signal-to-noise ratio tends to become saturated at high RF signal levels.
Therefore, a diversity reception system is needed that provides for improved receiver selection at both high and low RF signal input levels. The present invention allows for receiver selection at high and low RF input signal levels by selecting the receiver with the highest output signal-to-noise ratio when the receiver is delivering less than a threshold signal-to-noise ratio and selecting the receiver with the highest RF input signal level otherwise.